Being a former microbiologist, I find the idea that there is some generic way to treat "internal parasites" with one magical elixir a bit troublesome. Internal parasites could be protozoans (treated a certain way), they could be bacteria (an antibiotic may be needed, and one specifically designed to work on whether the phage is gram+ or gram-) .
An internal parasite might also be nematode(worm) and require a dewormer. Or even a crustacean that requires a chitin inhibitor to treat it. Each type parasite may require a different medication.
The link Big-G give a very good representation of the large picture and problem, and a microscope could help, but you have you have an idea of what are you looking for? Which is the parasitic one, the ciliate, the rod, the squiggly one? Just to see certain bacteria under a microscope, staining the slide is often needed to make them visible under a scope.
All animals have a large varied population of species that live in their digestive tract (bacteria, nematodes etc,) some are good, some bad.
There is always a certain ratio in the gut, but if a fish is under stress, event good ones can turn bad, and become pathogenic.
And just because a tank has sat "fish-less" for a few months doesn't mean something pathogenic is dead, many pathogens can go dormant for months, and wake up to infect as soon as water is added.
The bacteria that causes columnaris disease has that ability to remain viable in a small layer of dust.
Consider the natural pools in dry and tropical areas, that dry up seasonally, and where life returns with the rains.
Quarantining each new fish for 2 months or more, is the only realistic preventative. It often takes that long for a bacteria or parasite to attain enough numbers to become apparent.